K.,
As far as I could find out, Singulair hasn't been studied in nursing mothers. I would be a little leary of taking it. Is it possible to take another allergy med that has been studies, such as Zyrtex or Xyzal? Those have been okayed for breastfeedng mommies. I just worry because I know Singulair is used in Asthma patients and most asthma medicines aren't okay during breastfeeding. Anyhow, it probably will decrease your milk supply. I had to take fenugreek (you can find it at most stores) caplets to keep up my supply. Plus, drink lots (I mean LOTS) of water to keep well hydrated :)

I would not think it would affect milk supply. When going to their website, it says that it is not a decongestant (which would affect milk supply) but rather a leukotriene blocker. A decongestant would dry up everything in your body, a blocker just blocks your body's response to the allergen. Doesn't look like there is a lot of information out there on this specific drug, but I did find this on a forum for healthcare professionals:
http://66.230.33.248/discus/messages/44/2400.html

It looks like it is very similar to an antihistamine, which does not lower milk supply, just blocks histamine response. If you are interested in more info, look at the antihistamine section on this page:
http://www.kellymom.com/health/meds/cold-remedy.html#meds

It depends on the medication. You can use this website http://toxnet.nlm.nih.gov (from the federal government)to look up the info. Also, Dr Thomas Hale is considered the authority on meds and breastfeeding. Check his website at http://neonatal.ttuhsc.edu/lact/ . Hope you feel better soon.